Thermal inkjet hardcopy devices such as printers, large format plotters/printers, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1(February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes termed "dot locations", "dot positions", or "Pixels". Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet hardcopy devices print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the ink ejection element. When electric printing pulses activate the ink ejection element, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly-arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied by the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
In an inkjet printhead ink is fed from an ink reservoir integral to the printhead or an "off-axis" ink reservoir which feeds ink to the printhead via tubes connecting the printhead and reservoir. Ink is then fed to the various ink ejection chambers either through an elongated hole formed in the center of the bottom of the substrate, "center feed, " or around the outer edges of the substrate, "edge feed. " In center feed the ink then flows through a central slot in the substrate into a central manifold area formed in a barrier layer between the substrate and a nozzle member, then into a plurality of ink channels, and finally into the various ink ejection chambers. In edge feed ink from the ink reservoir flows around the outer edges of the substrate into the ink channels and finally into the ink ejection chambers. In either center feed or edge feed, the flow path from the ink reservoir and the manifold inherently provides restrictions on ink flow to the ink ejection chambers.
Color inkjet hardcopy devices commonly employ a plurality of print cartridges, usually two to four, mounted in the printer carriage to produce a full spectrum of colors. In a printer with four cartridges, each print cartridge can contain a different color ink, with the commonly used base colors being cyan, magenta, yellow, and black. In a printer with two cartridges, one cartridge can contain black ink with the other cartridge being a tri-compartment cartridge containing cyan, magenta and yellow inks, or alternatively, two dual-compartment cartridges may be used to contain the four color inks. In addition, two tri-compartment cartridges may be used to contain six base color inks, for example, black, cyan, magenta, yellow, light cyan and light magenta. Further, other combinations can be employed depending on the number of different base color inks to be used.
The base colors are produced on the media by depositing a drop of the required color onto a dot location, while secondary or shaded colors are formed by depositing multiple drops of different base color inks onto the same or an adjacent dot location, with the overprinting of two or more base colors producing the secondary colors according to well established optical principles.
In color printing, the various colored dots produced by each of the print cartridges are selectively overlapped to create crisp images composed of virtually any color of the visible spectrum. To create a single dot on paper having a color which requires a blend of two or more of the colors provided by different print cartridges, the nozzle plates on each of the cartridges must be precisely aligned so that a dot ejected from a selected nozzle in one cartridge overlaps a dot ejected from a corresponding nozzle in another cartridge.
The print quality produced from an inkjet device is dependent upon the reliability of its ink ejection elements. A multi-pass print mode can partially mitigate the impact of the malfunctioning ink ejection elements on the print quality. The concept of printmodes is a useful and well-known technique of laying down in each pass of the printhead only a fraction of the total ink required in each section of the image, so that any areas left white in each pass are filled in by one or more later passes. This tends to control bleed, blocking and cockle by reducing the amount of liquid that is on the page at any given time.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a "printmode." Printmodes allow a trade-off between speed and image quality. For example, a printer's draft mode provides the user with readable text as quickly as possible. Presentation, also known as best mode, is slow but produces the highest image quality. Normal mode is a compromise between draft and presentation modes. Printmodes allow the user to choose between these trade-offs. It also allows the printer to control several factors during printing that influence image quality, including: 1) the amount of ink placed on the media per dot location, 2) the speed with which the ink is placed, and, 3) the number of passes required to complete the image. Providing different printmodes to allow placing ink drops in multiple swaths can help with hiding nozzle defects. Different printmodes are also employed depending on the media type.
One-pass mode operation is used for increased throughput on plain paper. Use of this mode on other papers will result in too large of dots on coated papers, and ink coalescence on polyester media. In a one-pass mode, all dots to be fired on a given row of dots are placed on the medium in one swath of the print head, and then the print medium is advanced into position for the next swath.
A two-pass printmode is a print pattern wherein one-half of the dots available for a given row of available dots per swath are printed on each pass of the printhead, so two passes are needed to complete the printing for a given row.
Similarly, a four-pass mode is a print pattern wherein one fourth of the dots for a given row are printed on each pass of the printhead. Multiple pass thermal ink-jet printing is described, for example, in commonly assigned U.S. Pat. Nos. 4,963,882 and 4,965,593. In general it is desirable to use the minimum number of passes per full swath area to complete the printing, in order to maximize the printer throughput reduce undesirable visible printing artifacts.
The ability to achieve good tone scale is crucial to achieving photographic image quality. In the highlight region of the tone scale, nearly invisible dots and lack of graininess are required. Areas of solid fill require saturated colors, high optical density and no white space. Also, the ability to place more than one drop from a given printhead into a pixel is essential to achieving this photographic image quality. Another important attribute of an imaging system is high throughput.
Previous methods such as multi-pass printing described above put more than one drop from a given printhead in a pixel, but this is done on separate passes. The disadvantages of this approach are: (1) throughput is compromised because a separate pass is required for each drop placed from a given printhead onto a pixel, (2) in areas of high density printing, drops are put into every pixel on every pass which leads to dot coalescence which degrades image quality, and (3) it is an inefficient way to cover white space in the midtone regions of the tone scale where slight drop placement variations are required to fill in white space which is difficult when multiple drops are placed on a pixel in separate passes.
Another solution for achieving good tone scales is to use a six-ink printing system. This approach uses black ink, yellow ink, light cyan ink, dark cyan ink, light magenta ink and dark magenta ink. Good image quality is achieved in highlight regions by using only the yellow, light cyan and light magenta inks. The black, dark cyan and dark magenta inks are used in more saturated areas of the image. The disadvantages of this system are (1) the complexity of having a six-ink system (more inks, more complicated color maps and product cost and size) and (2) transitions that degrade image quality are observed in the tone scale when the dark cyan and dark magenta, which are highly visible, are first used.
Another approach to form different dot sizes is to use multiple drop volumes on the same printhead (See, U.S. Pat. No. 4,746,935). The primary disadvantage of this approach is the need for multiple drop generators which increases cost and complexity.
Even when using the above described methods and apparatus, the creation of crisp and vibrant images with accurate tone equal to those produced by conventional silver halide photography has not been achieved.
Due to the increasing use of digital cameras to produce digital images and the use of scanners to input conventional photographs into personal computers, the demand has rapidly increased for printers which can produce photographic quality prints from these images at high throughput.